In power converters, losses appear as none of the components has ideal characteristics. The losses introduce heat in the power circuitry, which apart from consuming energy introduces thermal strain to all components, reducing their life span.
It is desirable to increase the frequency of operation of power converters as the generated output then can be more exactly controlled. Increasing the switching frequency leads to lower switching ripple, and smaller component values which in turn leads to a more compact, light-weight and cost-effective implementation of the invention. Further, lower switching ripple allows for potentially lowered EMI, which aligns with the goal of a non-disturbing switch. Still further, having a high switching frequency allows for higher frequency currents to be generated by the power converter, widening the range of applications suitable for the converter.
However, increasing the frequency also increases switching losses, as most losses appear on switching cycle basis. Forcing the transistor to commutate while a current is flowing through it or when there is a potential difference over it, requires energy which must be supplied to the gate of the transistor. Thus, reducing the current through the transistor or the voltage thereover reduces the total power input for the switch and thus the total power input to the system.
One way of reducing the losses over a particular switch is to ad a resonant component to the circuitry in which a current is generated by an inductive element, by the discharge of a capacitor. A circuit employing this technique is known as a resonant converter, and the method of using resonance to facilitate commutation is known as soft switching. There are generally two types of soft-switching: low-voltage switching and low-current switching. Low-voltage switching involves minimizing the voltage or potential difference over the switch prior to commutation, whereas low-current switching involves minimizing the current through the switch prior to switching.
Generating less EMI noise is an important goal in its own right. In applications where the converter or inverter is connected directly to the grid, EMI noise can cause problems which are normally solved by employing EMC-filters. EMC-filters are placed in series with the converter, thereby handling the full current capacity. By minimizing the EMI, EMC filters can be eliminated from the converter design.
One soft switching solution is provided in U.S. Pat. No. 5,047,913 (to De Doncker et al.). De Doncker suggests using controlled switches in the resonant auxiliary circuitry for overcoming the problem of active device switching losses in power converters. The reduction of losses in the power converters enables operation at higher switching frequencies. De Doncker describes that the resonant output voltage may fall short of the opposite rail voltage due to component resistances, device conduction losses and inadequate forcing potential. As a result, the next switching device in the inverter pole to be turned on may be switched at the peak of the resonating voltage, and hence must absorb some switching losses due to the non-zero voltage turn-on, including the energy dump from the parallel capacitor.
The introduction of an auxiliary switch reduces the resistance that the main switches are facing at commutation, and thereby the losses in the switch. However, the transistor elements of the auxiliary switch still need to commutate with a potential difference thereacross. The insulating capabilities of the gate oxide, which is a thin insulating layer separating the gate from the underlying source and drain, provides the commutation resistance that the gate signal needs to affect to force the transistor to commutate. In a switch based power converter, the accuracy of the output current is dependent on the frequency of the switching, which means that a high switching frequency is advantageous. As a switch with high switching frequency and small losses is desired, a transistor that commutates with small gate signal should be provided, as the energy of the gate signal becomes losses in the form of heat developed in the transistor. For achieving these properties, the oxide layer is made very thin, which reduces the required amount of energy that the gate signal needs to supply for forcing the transistor to commutate.
A transistor has a maximum blocking voltage, i.e. the voltage the transistor will reliably withstand without breakdown when in the off-state. The maximum blocking voltage depends on the gate oxide. When the maximum blocking voltage is exceeded, there is a risk that the gate oxide fails and thus loses its insulating capabilities. This failure is known as oxide breakdown. The risk of oxide breakdown increases as the oxide layers are made thinner. One form of oxide breakdown is oxide rupture which is caused by a high voltage being applied across the oxide layer. The high voltage causes the thinnest spot in the oxide layer to exhibit dielectric breakdown and thus allow current to flow. The flowing current causes the oxide layer to heat up, which further enables current flow through the oxide layer causing a chain reaction eventually causing a meltdown of the semiconductor material and thus a short circuit in the transistor. It would therefore be advantageous to provide a circuit for a power converter which reduces the risk that the potential difference over the transistor element exceeds the maximum blocking voltage of the transistor.